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Due to the unique subsoil conditions prevailing in the Mexico City basin, seismic risk has been strongly correlated to site effects. Thus, during the Mw 8.1 09/19/1985 subduction fault earthquake, and its strong aftershock Mw 7.5 09/21/1985, extensive damage was observed in the area, along with the loss of thousands of lives, despite these events had an epicentral distance of around 430 km from Mexico City. The observed damage was mostly due to site affects originated by the high plastiCity clay deposits found in the basin, which lead to large amplifications, and duration elongation of the ground motions coming from the epicenter. In addition, a frequency content modification occurs, which in turn, leads to a double resonance effect between the incoming ground motions, soil deposits, and the damaged buildings. Exactly 32 years after this devastating event, the Mw 7.1 09/19/2017 normal fault earthquake, reminded us of the importance of accounting for site effects, and most importantly the need to carry out a proper characterization of basin geometry, soil profile configuration, hydraulic conditions, and maintenance-structure periodic assessments of the building stock in Mexico City. This is required to reduce uncertainties of seismic vulnerability studies for extreme-event seismic hazard scenarios. In this paper, the role of site response and seismic soil-structure interaction as key factors responsible of the observed damage in the City is revisited, through series of 3D finite difference models of typical structure-foundation-soil typologies found at the areas where most of the damage was observed, highlighting its clear impact in the final damage distribution observed around the City.

Ground subsidence due to underground water extraction leads to changes in dynamic properties such as shear wave velocity distribution and shear modulus degradation and damping curves, and more importantly, soil layer thickness and shape. These effects modify site predominant periods, frequency content, and spectral accelerations in both the free field and soil-structure systems during an earthquake. These variations can be substantial in highly compressible very soft clays, such as those found in the Mexico City basin. This paper examines the complex role that ground conditions, including settlement variability and structural tilting due to regional ground subsidence, had in the seismic performance of buildings during the Mw 7.1 2017 Puebla-Mexico City earthquake. A numerical study was undertaken to simulate the seismic response of conventional buildings supported by a compensate box-like foundation in soft clay, representative of those that exhibited major damage or collapse. Sets of three-dimensional finite difference models were developed with FLAC3D. Initially, the overall static behavior of the soil-structure system was assessed, considering both ground settlement and structure tilting due to ground consolidation, resembling the conditions prevailing right before the earthquake. Then, the seismic performance evaluation during the dynamic event was conducted at several consolidation times (i.e. 0, 25, 32 and 45 years). From the results gathered herein, it was clearly established the effect of the variation of the fundamental period of the soil deposit, and the initial ground deformation and structural tilting, on the seismic demand acting on the structure during the earthquake, which in turn, lead to most of the observed failures.